The present invention relates to the field of pyrotechnic initiators and deals more specifically with an electro-pyrotechnic initiator intended for motor vehicle safety.
Electro-pyrotechnic initiators are traditionally built from two electrically conducting pins which are held in place by overmoulded insulation and the upper ends of which are joined together by a heating resistive filament which is encased in an igniter bead consisting of a primary explosive such as lead trinitroresorcinate or lead styphnate and the lower ends of which are connected to a source of electrical current.
Such electro-pyrotechnic initiators are widely used to form devices for igniting pyrotechnic gas generators intended to inflate air bags to protect the occupants of a motor vehicle. In order to protect these initiators against parasitic electrostatic currents and avoid untimely undesired and possibly dangerous ignition, a ferrite core is generally placed between the pins, as described, for example, in the Patents U.S. Pat. No. 4,722,551 and EP-A-0,512,682. In order further to improve the ignition reliability, the heating resistive filament is sometimes replaced by a printed circuit comprising a resistive and heating part as described, for example, in Patent Application FR-A-2,704,944 or in its counterpart U.S. Pat. No. 5,544,585.
Whatever the case may be, systematically resorting to conductive pins appreciably increases the cost price and the complexity of manufacturing these electro-pyrotechnic initiators. What is more, the very frequent use of ferrite cores increases the external dimensions of the overmoulding surrounding the pins and does nothing to help reduce the overall volume of the pyrotechnic gas generators intended for motor vehicle safety.
Those skilled in the art are therefore still looking for electro-pyrotechnic initiators which are at the same time reliable in their operation, easy and economical to manufacture and small in volume.
The object of the present invention is specifically to provide such an initiator by dispensing with the pins and the ferrite cores and by integrating their functions into a complete printed circuit, contrary to all that has been done hitherto.
The invention therefore relates to an electro-pyrotechnic initiator connected to an electric cable consisting of two conductive wires and comprising:
i) a thermosensitive charge that can be initiated by a heating resistive element,
ii) an electric circuit connecting the said resistive element to the said conductive wires,
iii) means of electromagnetically protecting the said electric circuit,
characterized in that the said heating resistive element, the said electric circuit and the said means of electromagnetic protection are integrated into a circuit of conductive strips which are soldered to the said conductive wires and which are formed on a printed circuit support around which the said initiator is built.
The printed circuit support will normally consist of one of the insulating substances conventionally used for making such supports.
According to a first preferred embodiment of the invention, the said heating resistive element itself consists of a resistive strip formed on the printed circuit support and the thermosensitive charge is deposited on the said resistive strip.
The means of electromagnetically protecting the initiator against parasitic currents may consist of at least one filtering coil placed on the said circuit support. As a preference, in this case, the filtering coil may itself be printed on the said circuit support and, together with the electrical circuit, constitute a multi-layer printed circuit.
The said means of electromagnetic protection may alternatively consist of at least one capacitor arranged on the said circuit support.
According to a preferred embodiment of the invention, the said electric circuit and the said means of electromagnetic protection are covered by a layer of insulating varnish.
The circuit support, which may or may not be covered with a layer of insulating varnish, will moreover be partially coated with overmoulded insulation so that only that part of the circuit support that carries the heating resistive element covered with the thermosensitive charge is left free. When the thermosensitive charge is not itself at least partially held in place by the said overmoulding, it will advantageously be protected by a fragmentable mask. A mask made of very thin metal or a mask made of plastic, for example of transparent plastic, are suitable.
According to a second preferred embodiment of the invention, the said circuit support is in the form of a thin parallelepipedal card with two opposite flat faces. This second embodiment of the invention makes it possible to build electro-pyrotechnic initiators in which a separate circuit of conductive strips, each including a heating resistive element covered by a thermosensitive charge and means of electromagnetic protection is printed on each of the two flat faces of the circuit support. It is thus possible to build an initiator which, for a single electrical source, has two separate igniter heads and which is particularly dependable and reliable.
The invention also relates to an electro-pyrotechnic igniter, characterized in that an initiator according to the invention is surrounded by a fragmentable cap containing a granular igniter powder.
The invention finally relates to the use of an electro-pyrotechnic initiator according to the invention to ignite a pyrotechnic generator of gas 95 intended to inflate an air bag 96 to protect the occupant of a motor vehicle. The initiator may be used directly to ignite the pyrotechnic charge of the generator when the latter is in granular or pellet form, it will advantageously be used via an igniter like the one described above when the charge is in the form of a compact block.
The invention thus makes it possible, using small printed circuits which are very simple and very economic to produce, to construct initiators or igniters which are not very bulky and are very reliable.